Newton Ralpson on matlab

In matlab I am supposed to enter a starting guess ?0 to use as the first value for the Newton-Raphson method. If this guess causes the derivative of the temperature function to be zero (i.e. if ? ′ (?0 ) = 0 ), the method will immediately fail due to a division by zero. Check if the initial guess will cause the method to fail. If so, prompt the user to enter a new initial guess until the user enters a valid value. If the user does not enter an acceptable guess after 3 attempts, produce an error and terminate the program.

this is my code so far but somethings seems to be going can wrong can someone help me fix it

here is the commented code

f = @(x) (cos(x));

%

% fd = @(x) (-sin(x));

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% n=10;

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% i=1;

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% p0=0;

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% while(p0==0),

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% p0 = input('Enter the intial approximation: = ');

%

% end

%

%

%

% tol = 0.0001;

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% while(i<n),

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% d = f(p0)/fd(p0);

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% p0 = p0 - d;

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% if(abs(d) < tol),

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% break;

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% else

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% i = i+1;

%

% end

%

% end

%

% disp(['The Func. Value = ',num2str(p0)]);

[darova]

[matt noman]

comments

Answer 1

[David Hill]

You could try something like this:

function root = newtonR(f,fp,tol)
for k=1:3
    x=input('root guess');
    if ~isinf(1/fp(x))
        root=x-f(x)/fp(x);
        while abs(root-x)>tol
            x=root;
            root=x-f(x)/fp(x);
        end
        return;   
    end
end
xf='error';